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At birth, the infant brain weighs 2/3 to 3/4 pounds (300-350 grams) and contains 100 billion neurons. Connections in the parts of the brain that control basic survival and reflexes are already well-developed, and myelination in those areas is nearly complete.
The newborn begins a rapid period of brain growth. Synaptogenesis continues very rapidly, with neurons in all parts of the brain making trillions of connections. Myelination continues in most parts of the brain. The neurons controlling hearing and vision rapidly become myelinated.